The present invention relates to the inspection of glazings, in particular, to the inspection glazings for both distorting and non-distorting faults.
During production, the glass used in automotive glazings is inspected for various defects that may affect the optical quality of the finished glazing product. For example, the glass may have faults acquired through processing, such as, edge faults, brillantatura and shiners from the cutting and grinding processes used to cut the glass to size. Alternatively, faults may arise through distortion, thickness and curvature variations from the firing and bending processes used to shape the glass. For example, a secondary image may be seen when viewing an object through shaped glass. This is the case for both single ply and laminated glazings.
Faults may also arise within the body of the glass, from the glass-making process, or on the surface of the glass from processing, either during or after manufacture. For example, glass made using the float process may have small gas bubbles and nickel sulphide inclusions within the body of the glass, or areas where cutlet (recycled glass) has not melted properly, and formed fused regions. The surface of the glass may have tin specks, roller imprints (from the lehr rollers), abrasions, chips and chill cracks. Whilst some of these faults can be detected between the glass leaving the float line and further processing, such as firing and bending, some, such as abrasions, cannot be detected until a final inspection, as they may arise at any point in the processing of the glass. These faults have implications on the final quality, cosmetic appearance and durability of a glazing.
Typically, faults in glass are detected using optical inspection processes. These are where the glass is illuminated either in transmission or reflection, and the variations in transmitted light used to determine whether a fault is present. One well known quality inspection method is the shadowgraph. A glazing is positioned between a localised light source (a high intensity point source) and a screen, and a shadowgraphic image of the glazing is projected onto the screen, and recorded using a CCD (charge-coupled device) camera. The shadowgraph of a glazing is characterised by illumination variations that are related to the transmitted distortion of the glazing, caused by thickness variations in the glass. These thickness variations result in an effect known as “wedge”, where shallow, wedge-shaped sections of glass are seen, causing deflection of light resulting in optical distortion. Variations in the wedge angle result in light converging or diverging, giving the illumination variations of the shadowgraph. Shadowgraphs may be generated from flat or curved glazings, and may be incorporated as the final inspection step in a glass processing line.
Other optical techniques may also be used for the inspection of glass. Faults within the glass may be imaged using both bright-field and dark-field techniques. Bright-field techniques can be used to detect small faults (as little as 0.2 mm in size), both in focus, to detect non-distorting faults, and out of focus, to detect distortion. Dark-field techniques may also be used to detect small faults (as little as 0.2 mm in size). However, as all faults scatter light, images of distorting faults can be viewed in focus. Distorting faults may also be viewed using shadowgraph techniques.
Separate bright-field and dark-field measurements are needed to build up an accurate picture of the quality of a glazing. The images generated by each technique may be recorded using separate detectors and integrated in a processing step. Alternatively, both bright-field and dark-field image components of a single image may be combined onto a single detector. However, when this is done, the combination of a peak in the dark-field signal with a trough in a bright-field signal may result in a low combined response, as many faults which scatter light to produce a dark-field image will absorb light in a bright-field image. In addition, the dynamic range of the signals measured is compromised, and information is lost. Determining whether a fault is present or meets pass/fail criteria, is therefore complex, and false rejects may occur. Such methods are therefore difficult to implement successfully on a production line.
It is therefore desirable to find a way to integrate bright-field and dark-field imaging techniques effectively for use in an optical inspection method.